| Sumario: | Magnetic resonance imaging (MRI) offers superior soft-tissue contrast and potentially improved delineation accuracy as compared with computed tomography (CT), which is conventionally used for radiation therapy treatment planning. Unlike CT, there is no direct relationship in MRI between signal intensity and electron density, making dose calculation a challenge. Commercial solutions are now available that produce a synthetic CT (sCT) from MRI images that can be used for dose calculation and image verification. The aim of this project was to implement and evaluate a robust process using MRI simulation only for planning prostate Stereotactic Body Radiation Therapy (SBRT) cases, eliminating the need for a CT simulation for dose calculation. A 5 stage implementation and evaluation process was developed by the team to systematically transition all prostate SBRT cases to MRI only planning. Phase 1 (Pre-Initiation phase) included predicate validation work conducted primarily by physics. In this phase, 10 cases were selected to evaluate the accuracy of sCT generation and subsequent dose calculation. Phase 2 (Process Initiation) was primarily focused on workflow with the development of processes and data pathways. Five cases were utilized for this phase to establish the planning workflow in terms of data transfer and evaluate feasibility of contouring by physicians and radiation therapists for creation of protocol for parallel CT/MRI planning. For Phase 3 (Process Development), MRI only plans were generated for 15 cases with a backup CT planning done for verification purposes. Dosimetric data was compared between the plan generated from sCT and the plan generated from CT. For the MRI only plans, issues with patient marking were identified and resolved, the ability to identify fiducial markers on sCT was assessed and in terms of image guidance for treatment (IGRT), the utility of automatic registration tools was evaluated and IGRT guidelines were refined. Phase 4 (Process Prototyping) included 20 cases that with planning with MRI only but all patients had a CT simulation as an unused data set that could be used in the event of any unforeseen issues. Phase 5 (MRI only Simulation) will completely eliminate the CT simulation and see contouring and planning on sCT as routine practice. This phase requires broader staff training, protocol updates and continuous process monitoring and improvement. • Soft tissue definition is excellent • Patients will have single simulation appointment, reducing the time they have to hold a relatively full bladder • The sCT and MRI images are acquired at same time and share the same coordinate system which produces less variation with image registration • Some cases are not suitable for MRI planning due to contraindications to MRI and a process needs to be established to identify these cases early in the referral process to avoid unnecessary rework • Establishing a process to tattoo patients in an MRI environment and identification of MRI visible skin markers • Additional training of staff may be required on the differences in contouring on MRI images compared to CT • Impact on IGRT protocols; although the sCT is of a similar quality to regular CT, fiducial markers appear as signal deficits and must be matched to contours instead of the image itself. Phase 5 implementation is expected for February 2023 with all suitable prostate SBRT cases planned using MRI only. This process will be used for implementation in other sites identified for MRI only planning.
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